1. Field of the Invention
The present invention relates to a boat propulsion system and a method for controlling a boat propulsion system.
2. Description of the Related Art
There are boats equipped with a plurality of boat propulsion units in order to improve high-speed performance, turning performance, steering stability, and other boat performance factors. An operation device capable of outputting operational commands at least in the directions of forward, reverse, left, and right is equipped in the boat in order to facilitate, even for a user without skill in operating a boat, operation of a boat provided with a plurality of boat propulsion units.
For example, Japanese Laid-open Patent Application No. 2005-319967 discloses a boat in which two propulsion units are operated by a joystick. In this boat, two propulsion units are controlled so that the boat is moved laterally or rotated based on the operational command provided by joystick.
Japanese Laid-open Patent Application No. 09-156596 discloses a boat equipped with four propulsion units. In this boat, the inside two of the four propulsion units are controlled so that the boat is moved or rotated based on the operational command provided by the joystick. However, the outside two propulsion units are auxiliary propulsion units and are not steered.
The following problems arise when the control in the two-engine boat of Japanese Laid-open Patent Application No. 2005-319967 is applied without modification to a four-engine boat when the boat is made to move laterally on the basis of an operational command provided by an operation device in a boat equipped with four propulsion units. In order to cause a boat 100 to move laterally, the intersection of the lines of action of the propulsion forces generated by four propulsion units 101 to 104 must all match a resistance center RC, as shown in FIG. 10. However, since there is a limit to the steering angle of the propulsion units, there are cases in which the outside propulsion units 101 and 104 cannot be adequately steered such that lines of action L101 and L104 pass through the resistance center RC.
Thus, it is possible to steer only the two inside propulsion units to move laterally, as shown in Japanese Laid-open Patent Application No. 09-156596. However, in this case, sufficient propulsion forces cannot be generated for a relatively large boat equipped with four propulsion units, because the propulsion forces in the lateral direction are low.